A conventional image projector is constructed such that the principal ray in the center of the screen impinges upon the screen approximately perpendicularly thereto, and it can be sized thinner by reducing the depth dimension or the dimension perpendicular to the screen. Accordingly, a rear mirror having no power is provided behind the screen, and the projected light is once reflected at the rear mirror and then projected onto the screen.
With such a configuration, the image projector may be sized still thinner by using a wider-angle optical system or by placing the rear mirror closer to the screen in parallel thereto, but these approaches are not successful because they cause interference between the projecting optical system and the projected light.
In a structure proposed to solve this problem, a still thinner image projector is realized by using an off-axis projecting optical system with a wider angle and shorter focus, while avoiding interference in the projecting optical path.
As compared with central-projection image projectors, this structure requires a projecting optical system with an extremely large angle of projection view, and it is advantageous to use reflecting mirrors causing no chromatic aberration, in order to reduce chromatic aberration caused by the wider angle. Accordingly, optical systems formed only of a plurality of reflecting mirrors are proposed, and optical systems formed of a group of refracting lenses and a reflecting mirror having a power are also proposed, as described in Patent Document 1 and Patent Document 2, for example.
Also, when a refracting Fresnel lens is used to form the screen, the screen exhibits deteriorated light transmittance in regions of larger angle of projection view, while using a total-reflecting Fresnel lens to form the screen results in deteriorated light transmittance in regions of smaller angle of projection view. Accordingly, a method is proposed which uses a hybrid structure having both of refracting and total-reflecting portions, in order to ensure appropriate light transmittance of the screen, as shown in Patent Document 3, for example.
Patent Document 1: Japanese Patent Application Laid-Open No. 2002-207168 (FIG. 39)
Patent Document 2: International Publication WO 01/06295 (FIG. 20)
Patent Document 3: International Publication WO 02/27399 (FIG. 7)
When only a plurality of reflecting mirrors are used to realize a thinner image projector, it is difficult, since mirrors are reflecting components, to arrange the plurality of mirrors without causing interference in the optical path, and then the optical path must be bent complicatedly.
Also, mirrors are more sensitive than refracting lenses to positional and angular displacements, and therefore a complicated configuration is required to highly precisely position all of the plurality of mirrors.
Furthermore, in a structure using both of a group of refracting lenses and a mirror, realizing a wider angle with large negative power of the mirror requires that the group of refracting lenses be configured complicatedly in order to correct large curvature of field caused by the mirror.